1. Field of the Invention
The present invention relates to an inkjet printing apparatus that agitates ink in an ink reservoir and a method for agitating ink.
2. Description of the Related Art
Mainly dye inks and pigment inks are used for a printing apparatus performing printing by an inkjet method. Dye inks have high-color reproducibility, and have excellent characteristics such as gloss and high-quality color development. Further, dye inks are dissolved in solvents, which gives the advantage that disproportioned ink concentration does not easily occur. However, because color materials in dye inks are on the molecular level, dye inks are easily decomposed by light or an active gas, which results in a problem of lack of weather resistance.
Pigment inks have excellent resistance to weather as compared with dye inks because color materials diffuse in clumps in solutions. Pigment inks have had a reputation of being inferior in reproducibility and color developing compared to dye inks. However, the quality of pigment inks has been improved due to improvement of ink materials and the like in recent years, and pigment inks have come into greater use for photographic printing and the like in the same way as dye inks.
Pigment inks have a problem in that, because the color materials thereof diffuse in solutions, when the inks are left untouched for a long time, the color materials in the inks settle out, which results in a concentration difference generated in an ink tank. To prevent such a problem, a method to provide uniform ink concentrations so as to prevent a concentration difference has been proposed. U.S. Pat. No. 6,695,441 discloses a method for vibrating ink in an ink tank at a predetermined time with an ultrasonic transducer. Settling-out of the ink is suppressed and a concentration difference in the ink tank is suppressed by periodically vibrating the tank.
U.S. Pat. No. 6,951,382 discloses an inkjet printing apparatus in which ink in an ink tank is agitated by causing a print head to scan without ejecting the ink at predetermined times.
However, even if the ink in the tank is agitated at predetermined times by using a timer in the above-described method, there is a risk that the agitation is performed more than necessary, which wastes time for processes other than printing. For example, when printing is performed by the printing apparatus, even when the ink is sufficiently agitated by causing the print head to scan, there is a possibility that the ink may be agitated over the predetermined time immediately thereafter. In this case, there is a risk that the agitation is carried out for a long time regardless of the fact that the ink has already been sufficiently agitated by the print head scan. Such agitation is not really necessary, and results in too much wasteful agitation.
Further, agitation based on the timer is carried out when the printing apparatus is not performing printing. That is, while the ink in the ink tank is being agitated, it is impossible for a user to perform printing. While agitation is carried out, it is necessary for the user to wait for the completion of the agitation. Thus, the longer the time for the agitation, the lower the productivity of printed matters by the printing apparatus.